1. [Field of the Invention]
The present invention relates to a starter for starting an engine.
2. [Description of the Prior Art]
Up to the present time, a starter (a coaxial type starter) coaxially arranging an electromagnetic switch, an over-running clutch provided with a pinion to mesh with a ring gear, a plunger (movable core) on an output shaft is known.
This type of starter operates as shown below.
That is, when current is sent to an exciting coil of the electromagnetic switch, the plunger is attracted by an exciting core of the electromagnetic switch and after a short period when the plunger begins to move, a movable contact and a stationary contact are brought into contact and electric power is supplied to a DC motor so that the output shaft is rotated by way of a motor shaft and a reduction mechanism. Then, the over-running clutch that is spline-connected to the output shaft moves toward the ring gear, the pinion and the ring gear mesh and the engine is started.
The plunger is formed in a tubular shape, arranged so as to cover the outer circumference of the output shaft and move the over-running clutch toward the ring gear.
Further, the over-running clutch is a so-called one-way clutch having a thrust spline that has a tubular portion with a helical spline formed on the inner circumference for spline-connection with a helical spline formed on the output shaft.
In the above-mentioned starter, the output shaft and the thrust spline of the over-running clutch that is spline-connected to this output shaft are generally formed from hardened steel for strength.
However, when the output shaft and the thrust spline are formed from the hardened steel, considerable magnetic flux leaks to the output shaft from the plunger and to the thrust spline from the output shaft. In addition, when the exciting core of the electromagnetic switch begins to attract the stationary state plunger, the attracting force (the initial attracting force) will become insufficient. That is, there has been a problem whereby the attracting force to the plunger becomes insufficient.
In the above-mentioned case, it would be possible to reduce the leakage of magnetic flux by providing a large distance between the output shaft and the plunger (the distance between the inner circumference of the plunger and the outer circumference of the output shaft (air gap)); however, this creates a problem inasmuch as the size of the starter increases in the radial direction large and as a result, the overall starter size is too great.
For instance, Japanese Published Unexamined Patent Application No. 31992-1996 disclosed that an output shaft formed using non-magnetic material (generally, material having a relative permeability xcexc lower than 1.02, for instance, such as stainless steel, brass, resin, etc.) is used so as not to disturb the magnetic field around an electromagnetic switch.
According to this, hardly any magnetic flux leaks to the output shaft and it becomes possible to improve the attracting force to the plunger. Further, it is possible to reduce the distance between the plunger and the output shaft. Therefore, the size in the radial direction can be reduced and the size of the entire starter can be reduced.
However, there has been a problem wherein an output shaft and a thrust spline of an over-running clutch are spline-connected and strong pressure is applied to the output shaft and a helical spline portion of the thrust spline when operating a starter as mentioned above, therefore, if an output shaft and a thrust spline formed with non-magnetic material are used, there is a strength-related problem.
Further, as mentioned above, if the output shaft and the thrust spline formed using hardened steel are used, there is no problem in connection with strength; however, considerable magnetic flux leaks to the output shaft and the thrust spline, and the attracting force to the plunger becomes insufficient. In this case, a problem is created wherein in order to assure a sufficient attracting force to the plunger, the starter becomes large in size.
The present invention was made to solve such problems as mentioned above and its object is to provide a starter capable of securing strength of an output shaft as well as a thrust spline, reducing leakage of magnetic flux and assuring sufficient attracting force to a plunger.
Another object is to provide a starter that can be made small in size.
At least one of an output shaft and a thrust spline of a starter of the present invention is formed from magnetic material having a lower relative permeability than iron.
Further, at least one of an output shaft and a thrust spline is formed from heat-treated martensitic stainless steel.
Further, at least one of an output shaft and a thrust spline is formed from hardened Austenitic stainless steel material.
The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.